In the recent years, it has been shown that it is possible to tune the structural and functional properties of metallic nanostructures in the form of thin films and nanoporous structures; i.e., structures having an extremely high surface-to-volume ratio.
The basic idea of the tunability of the electronic structure and properties of nanocrystalline materials was first described by H. Gleiter, J. Weissmuller, O. Wollersheim and R. Würschum, and published by them in Nanocrystalline Materials: A way to solids with tunable electronic structures and properties?, Acta Mater. 49 (2001), 737-745.
DE 199 52 447 C1 and J. Weissmüller, R. N. Viswanath, D. Kramer, P. Zimmer, R. Würschum and H. Gleiter in Charge-Induced Reversible Strain in a Metal, Science 300 (2003), 312-315, describe that reversible length changes of nanoporous gold can be achieved by accumulating electric charges at the interface of an electrolyte with the metallic nanostructures in a double layer.
C. Bansal, S. Sarkar, A. K. Mishra, T. Abraham, C. Lemier and H. Hahn described in Electronically tunable conductivity of a nanoporous Au—Fe alloy, Scripta Materialia 56 (2007), 705-708, that reversible changes in the electrical resistance of nanoporous gold and nanoporous Au—Fe alloys can be achieved by accumulating electric charges at the interface of an electrolyte with the metallic nanostructures in a double layer. The changes in the electrical resistance observed at applied charge densities of approximately 50 μC/cm2 were in the range of a few percent.
In Electrically Tunable Resistance of a Metal, Phys. Rev. Lett. 96 (2006), 156601, M. Sagmeister, U. Brossmann, S. Landgraf, and R. Würschum described the same effect at the same order of magnitude for nanoporous Pt.
The concept of tunability of the electrical conductivity in semiconductors, such as Si and Ge, which is known as “electric field gating”, has been established for decades and forms the basis for their use in electronics. The basic structure consists in an arrangement including a source and a drain coupled together by the semiconducting material, and a gate electrode isolated from the semiconductor by a suitable gate oxide. The conductivity of the semiconducting layer can be varied over a wide range by a gate voltage applied to the gate.
In the case of oxides, the region that can be influenced by interface charges; i.e., the space charge region, is much larger than in the above-mentioned metals, but smaller than in the case of semiconductors, which have a lower charge carrier density than conductive oxides. To date, however, hardly any studies have been conducted on the use of oxides as tunable materials.
As regards the functional properties of conductive oxides, R. Misra, M. McCarthy, and A. F. Hebard described the ability to control the electrical conductivity of a thin indium oxide layer in an ionic liquid, and to measure sheet impedance changes greater than four orders of magnitude, in Electric field gating with ionic liquids, Applied Physics Letters 90, (2007) 052905.